title: Mercury cycling in agricultural and managed wetlands of California, USA: seasonal influences of vegetation on mercury methylation, storage, and transport.
abstract: Plants are a dominant biologic and physical component of many wetland capable of influencing the internal pools and fluxes of methylmercury (MeHg). To investigate their role with respect to the latter, we examined the changing seasonal roles of vegetation biomass and Hg, C and N composition from May 2007-February 2008 in 3 types of agricultural wetlands (domesticated or white rice, wild rice, and fallow fields), and in adjacent managed natural wetlands dominated by cattail and bulrush (tule). We also determined the impact of vegetation on seasonal microbial Hg methylation rates, and Hg and MeHg export via seasonal storage in vegetation, and biotic consumption of rice seed. Despite a compressed growing season of ~3months, annual net primary productivity (NPP) was greatest in white rice fields and carbon more labile (leaf median C:N ratio=27). Decay of senescent litter (residue) was correlated with microbial MeHg production in winter among all wetlands. As agricultural biomass accumulated from July to August, THg concentrations declined in leaves but MeHg concentrations remained consistent, such that MeHg pools generally increased with growth. Vegetation provided a small, temporary, but significant storage term for MeHg in agricultural fields when compared with hydrologic export. White rice and wild rice seeds reached mean MeHg concentrations of 4.1 and 6.2ng gdw(-1), respectively. In white rice and wild rice fields, seed MeHg concentrations were correlated with root MeHg concentrations (r=0.90, p<0.001), suggesting transport of MeHg to seeds from belowground tissues. Given the proportionally elevated concentrations of MeHg in rice seeds, white and wild rice crops may act as a conduit of MeHg into biota, especially waterfowl which forage heavily on rice seeds within the Central Valley of California, USA. Thus, while plant tissues and rhizosphere soils provide temporary storage for MeHg during the growing season, export of MeHg is enhanced post-harvest through increased hydrologic and biotic export.
authors.affiliation: U.S. Geological Survey, Western Region Bureau of Regional Research, 345 Middlefield Road, MS 480, Menlo Park, CA, 94025, USA. Electronic address: lwindham@usgs.gov.
authors.forename: Lisamarie
authors.initials: L
authors.lastname: Windham-Myers
authors.affiliation: U.S. Geological Survey, Western Region Bureau of Regional Research, 345 Middlefield Road, MS 480, Menlo Park, CA, 94025, USA. Electronic address: mmarvin@usgs.gov.
authors.forename: Mark
authors.initials: M
authors.lastname: Marvin-DiPasquale
authors.affiliation: U.S. Geological Survey, Western Region Bureau of Regional Research, 345 Middlefield Road, MS 480, Menlo Park, CA, 94025, USA. Electronic address: kakouros@usgs.gov.
authors.forename: Evangelos
authors.initials: E
authors.lastname: Kakouros
authors.affiliation: U.S. Geological Survey, Western Region Bureau of Regional Research, 345 Middlefield Road, MS 480, Menlo Park, CA, 94025, USA. Electronic address: jlagee@usgs.gov.
authors.forename: Jennifer L
authors.initials: JL
authors.lastname: Agee
authors.affiliation: U.S. Geological Survey, Western Region Bureau of Regional Research, 345 Middlefield Road, MS 480, Menlo Park, CA, 94025, USA. Electronic address: lkieu@usgs.gov.
authors.forename: Le H
authors.initials: le H
authors.lastname: Kieu
authors.affiliation: U.S. Geological Survey, Fort Collins Science Center, Building 21, MS 963, Denver, CO, 80225, USA. Electronic address: cstricker@usgs.gov.
authors.forename: Craig A
authors.initials: CA
authors.lastname: Stricker
authors.affiliation: U.S. Geological Survey, California Water Science Center, 6000J St, Placer Hall, Sacramento, CA, 95819, USA. Electronic address: jafleck@usgs.gov.
authors.forename: Jacob A
authors.initials: JA
authors.lastname: Fleck
authors.affiliation: U.S. Geological Survey, Western Ecological Research Center, Dixon Field Station, 800 Business Park Drive, Suite D, Dixon, CA, 95620, USA. Electronic address: jackerman@usgs.gov.
authors.forename: Josh T
authors.initials: JT
authors.lastname: Ackerman
mesh_terms: D000383:Agriculture; D002140:California; D004784:Environmental Monitoring; D008628:Mercury / Q000032:analysis*; D008745:Methylation; D008767:Methylmercury Compounds / Q000032:analysis; D012621:Seasons; D014874:Water Pollutants, Chemical / Q000032:analysis*; D053833:Wetlands
chemical_list: D008767:Methylmercury Compounds; D014874:Water Pollutants, Chemical; D008628:Mercury
keywords: Bioaccumulation; Biomass; Litter; Methylmercury; Rice; Root